In laser thermal writing, the optical power of lasers is used to affect media as, for example, by a transfer of dye/ink from donor media to a receiver media. In order to enhance printing speed, high power density is needed at the media. One way to achieve high power is to use a number of independently modulated diode lasers. One such system is described in U.S. Pat. No. 5,109,460 to Baek assigned to Eastman Kodak Company, where each of the lasers is coupled to an optical fiber. The fibers are aligned on a v-grooved silicon substrate to form a fiber head which is then imaged onto the media using a printing lens. Another system is disclosed U.S. Pat. No. 5,453,777 assigned to Presstek, in which a number of pigtailed fibers are used where the fiber ends are arranged with spacings of 1 inch on the printing head, and each of the fibers is imaged by a dedicated printing lens onto the media.
High power laser light spots achieved by the above systems can affect the media either by the process of dye transfer, dye ablation or other light/media interaction mechanisms.
In some applications it is important to accurately control the printing process. For example, in laser thermal proofing systems, which use the process of laser thermal dye transfer, the density of the final print has to be very tightly controlled.
When the printing spots are not smooth in terms of their spatial distribution, occasional localized ablation can occur. The exact onset of the ablation process is not as well controlled as is the dye transfer process.
Variations in print density can then occur since more dye is typically transferred in the ablation process as compared with the dye transfer mode.
Also, when an array of spots produced by the printing head at the media is such that there are significant gaps between the spots, it may be beneficial to spread the light from each of the spots even when each one of them is an ideal uniform spot so as to fill up the gaps and prevent uneven printing.
In the case of fiber based systems, it is desirable to achieve a uniform illumination at the end of the fiber. However, in many cases the laser light has a measure of coherence which can produce laser speckle at the fiber ends. The spots pattern is highly dependent on the propagation through the fiber and can shift and change its shape as the fiber is moved such as when the printing head is translated in one direction during the process of printing.
One way to control and minimize the hot spots at the end of the fibers is to use a highly incoherent laser illumination by using as many fiber modes as possible. However by distributing the laser light into many fiber modes, the numerical aperture at the end of the fiber may increase and with it the depth of focus at the media will decrease. A short depth of focus is hard to maintain, and it commonly requires the use of special auto focus mechanisms.